Self-expanding medical prostheses, frequently referred to as stents, are well known and commercially available. They are, for example, disclosed generally in U.S. Pat. No. 4,655,771 (Wallsten), U.S. Pat. No. 5,061,275 (Wallsten et al.), and U.S. Pat. No. 5,645,559 (Hachtmann et al.). Stents are used within human body vessels for a variety of medical applications. Examples of such applications include treating stenoses, maintaining fluid openings in the urinary, biliary, tracheobronchial, esophageal, renal tracts, and vena cava filters.
It is preferable to minimize acute and chronic trauma to the luminal wall when implanting an intraluminal stent. A stent that applies a gentle radial force against the wall, and that is compliant and flexible when subjected to lumen movements, is preferred for use in diseased, weakened, or brittle lumens. Preferably, a stent is further capable of withstanding radially occlusive pressure from tumors, plaque, tissue hypertrophy and luminal recoil and remodeling.
A delivery tool that retains the stent in its radially compressed state is often used to present the stent to a treatment site within the body through tracts, lumens or vessels. The flexible nature and reduced radius of the radially compressed stent enables delivery to the treatment site through relatively small and curved tracts, lumens or vessels. In percutaneous transluminal angioplasty, an implantable endoprosthesis is introduced through a small percutaneous puncture site, airway, or port and is passed through such body vessels to the treatment site. After the stent is positioned at the treatment site, the delivery tool is actuated to release the positioned stent. The stent is allowed to self-expand within the body vessel. The delivery tool is then removed from the patient, and the stent remains in the vessel at the treated site as an implant. Typically, the delivery tool is designed for single use and is discarded.
Stents can be loosely grouped into three categories: metal, radially expanding polymeric, and fenestrated or laser-cut polymeric. Metal stents do not degrade in vivo and are, therefore, used when a permanent stent is desired. Polymeric stents are typically used to artificially and temporarily keep a lumen open. Most polymeric stents begin to degrade when exposed to water and are eventually excreted or sloughed off by the body. They do not require subsequent removal surgery. The expanding polymeric stents are comprised of a plurality of independent, woven fibers. Fenestrated stents are typically formed by providing a solid tube of polymeric stent material and using a laser to cut a pattern of holes through the wall of the tube in order to create a mesh appearance.
Each of the three types of stents have their own advantages and disadvantages. Metal stents do not degrade and maintain their strength indefinitely. Further, most metal stents do not experience significant “creep” or plastic deformation when stored in a compressed state over extended periods of time. However, as metal stents are permanent, removal requires an additional surgery. Polymeric stents are advantageous because they degrade, as discussed above. However, polymeric stents do experience creep and, therefore, may become useless if stored in a compressed state over time. Nonetheless, most polymeric stents are heretofore supplied pre-loaded in a stent delivery device and, therefore, must be used before creep occurs.
The two aforementioned types of polymeric stents exhibit some differences in performance and cost. Woven polymeric stents expand radially and shorten significantly when longitudinally compressed. Fenestrated polymeric stents resist radial expansion and longitudinal compression. Fenestrated stents are often extremely expensive, however. This expense is due to the intricate cutting that must be done with a laser to create the desired pattern. Not only is the cutting time-consuming, it results in a significant amount of wasted polymeric material. The polymeric material used to make stents is extremely expensive.
In addition to the problems associated with creep, described above, there is an additional problem with the present manner in which stents are packaged with a delivery device. Due to increasing health care costs, there is a growing demand for medical devices that can be sterilized and reused. However, as stated above, stent delivery instruments are supplied pre-loaded with a stent and are prepared for single use and disposal. This approach is not cost effective. Equipment and techniques for loading stents into a delivery tool have not yet been introduced that allow medical practitioners to load stents into delivery tools in a pre-operating room environment. Moreover, the appropriate and successful introduction of a sterilizable and reusable delivery tool requires an easy and effective loading device and a fool-proof method of use.
Thus, there remains a need for a stent delivery system that is capable of storing a stent in an expanded state until ready for placement, at which point the system permits the stent to be compressed and transferred into a delivery tool. There is also a need for a sterilizable and reusable stent delivery system which is easy to operate.